Fix ThreadPoolMergeExecutorServiceTests testIORateIsAdjustedForRunningMergeTasks

muted-tests.yml

@@ -353,9 +353,6 @@ tests:
 - class: org.elasticsearch.xpack.test.rest.XPackRestIT
   method: test {p0=transform/transforms_start_stop/Test schedule_now on an already started transform}
   issue: https://github.com/elastic/elasticsearch/issues/120720
-- class: org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceTests
-  method: testIORateIsAdjustedForRunningMergeTasks
-  issue: https://github.com/elastic/elasticsearch/issues/125842
 - class: org.elasticsearch.xpack.test.rest.XPackRestIT
   method: test {p0=transform/transforms_start_stop/Verify start transform creates destination index with appropriate mapping}
   issue: https://github.com/elastic/elasticsearch/issues/125854

server/src/test/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorServiceTests.java

@@ -281,8 +281,11 @@ public void testIORateIsAdjustedForRunningMergeTasks() throws Exception {
                     doAnswer(mock -> {
                         currentlyRunningMergeTasksSet.add(mergeTask);
                         // wait to be signalled before completing
-                        runMergeSemaphore.acquire();
-                        currentlyRunningMergeTasksSet.remove(mergeTask);
+                        try {
+                            runMergeSemaphore.acquire();
+                        } finally {
+                            currentlyRunningMergeTasksSet.remove(mergeTask);
+                        }
                         return null;
                     }).when(mergeTask).run();
                     doAnswer(mock -> {